Lignin -based hydrogels through a green-metrics lens: solvents, crosslinking chemistries, and scalable processes

Abstract

The sustainability of lignin-based hydrogels should be demonstrated rather than presumed from bio-origin alone. This critically aligns their contemporary synthesis and crosslinking routes (dynamic covalent chemistries (imine/boronate/arylhydrazone), metal–phenolic coordination, and enzyme-catalyzed and visible-light-initiated curing) with the 12 principles of green chemistry, using harmonized metrics and declared system boundaries. Across representative formulations, we benchmark PMI (typically 20%–80% excluding water/20%–200% including water), E-factor (simple vs. comprehensive), energy intensity (0.1–1.5 kWh kg⁻¹ gel for photo/dynamic routes vs. 2–6 kWh kg⁻¹ for thermal route), solvent rankings (CHEM21), GHS-based hazards, and end-of-life options. For antifreezing ion-hydrogels, we quantify performance-greenness trade-offs: σ = 0.05–0.5 S m⁻¹ at −20 °C with 5–20 wt% salt or deep eutectic solvent (DES), often accompanied by increased leachable burden unless recovery is >80%. This review highlights the hidden costs of metal salts and nanofillers (extractables and recovery) and prioritizes metal-free or readily recoverable catalysts and water/low-hazard media. Particular attention is given to industrial lignin heterogeneity (source, S/G/H ratio, and ash/metal content) and to greener fractionation/derivatization pathways that narrow dispersity and enable milder, lower energy gelation at scale. The review delivers a design-for-greenness scorecard and a decision framework that favour ambient-temperature processing, catalytic/enzymatic crosslinking, solvent/reagent recycling, and design-for-disassembly. This review proposes reporting standards for feedstock analytics, explicit PMI/E-factor boundaries, solvent-recovery yields, regeneration energy, leachate/ecotoxicity tests, durability and degradability protocols, and performance-per-impact metrics (e.g., mF cm⁻² per MJ and sorption capacity (q_e) per L solvent loss). Remaining gaps in long-term safety, quality control and field-scale reliability are identified, outlining a roadmap toward regulation-ready, commercially viable lignin hydrogels.